1. Field of the Invention
The present invention relates to a device and process for determining operating conditions of a control valve including a valve element operated by a pressure operated drive, the drive being controlled by a position controller and, more particularly, to a device and process for determining the condition of a seal at a point at which the valve element enters the valve chamber.
2. Description of the Prior Art
Control valves are well known in the art. For example, German Patent No. DE 42 33 301, which is incorporated in its entirety herein by reference, describes a control valve in which the position controller continuously monitors the operation of the valve. The system described by this patent includes a valve element, a pneumatic drive and a position controller. The pneumatic drive is connected to the valve element by a spindle and is also connected to the position controller. The drive moves the valve element which is located within a valve chamber via the spindle connection. The position controller controls the pneumatic drive and thus the movement of the valve element by supplying the drive with pressurized air as a correcting variable. The drive then moves the valve element via the spindle connection in response to receipt of the pressurized air. The correcting variable is supplied by the position controller in response to an electrical command signal of, for example, 4 to 30 mA received by the position controller. The position controller is also connected to the spindle and is able to determine the actual position of the valve element by sensing movement of the spindle. The position controller includes a sensor and built-in microprocessor serving exclusively to monitor the control performance of the valve. The movement sensed by the sensor is delivered to the microprocessor wherein the values received are integrated over time and compared with reference values to determine the control performance of the valve. Problems exist with such a device as, by in sensing only the valve movement to determine the control performance of the valve, many parameters, including mechanical parameters, influence the control performance of the valve and are not compensated for by such measurements.
At the entry point of the spindle into the valve chamber is a dynamic seal. This dynamic seal is often designed as a stuffing box. The seal must be absolutely tight and remain tight throughout operation of the valve. As this dynamic seal must remain tight it will exert a frictional force on the spindle thus hindering its movement and the movement of the valve element within the valve chamber. It is also very important to be able to monitor the condition of the seal. It is possible to monitor the condition of the seal by determining whether a change in the stuffing box frictional force upon the spindle has occurred.
Several attempts have been made determining this frictional force and whether a change in this force has occurred. In one instance a curve of the drive pressure has been plotted against the position of the spindle. The frictional force is then obtained by determining the hysteresis of the upward and downward movement of the spindle. Measurements taken in this manner require an additional sensor, namely a pressure sensor, in the drive which greatly increases both the power consumption and cost of the device. The additional power consumption needed by such devices is a critical factor due to the use of such valves in large chemical plants in which the amount of power consumption is highly regulated and in which there is a danger of explosions occurring during operation of the valves.
It is thus desirable to provide a control valve which does not require the high power consumption of the above mentioned valves and is also able to monitor the condition of the dynamic seal in an effective manner.